(1 ) Field of the Invention
The present invention relates to a technique for controlling cell communication in an asynchronous transfer mode (ATM) in an access network in which an optical subscriber line is shared by a plurality of subscribers, and particularly to a method for controlling an ascending transmission speed from a plurality of optical network units to an optical line termination, and a communication system therefor in case of providing a best effort type service which does not guarantee a subscriber""s available communication speed in a congested state of the network in a passive optical network (PON) system constituted of an optical line termination at a base station side. and of a plurality of optical network units.
(2) Related Art
FIG. 8 is a block diagram showing a constitution of a common PON system to be connected to an ATM network.
The PON system shown in FIG. 8 is an access network in which optical subscriber lines provided by branching a single optical fiber such as by an optical splitter are shared by a plurality of subscribers. Namely, the system has a basic constitution including a single optical line termination (OLT) 2 connected to an ATM network 1, and a plurality of optical network units (ONU""s) 4 connected to the optical line termination 2 via optical splitter 3. The OLT 2 accommodates a plurality of ONU""s by a single PON interface, while each of the ONU""s 4 accommodates a single or plural user terminal(s) 5.
In the above described PON system, those descending data from the OLT 2 are broadcast toward the respective ONU""s 4 in a time-division multiplexing manner, and each of the applicable ONU""s 4 takes out relevant information addressed to itself from the descending data. Meanwhile, concerning the ascending data from the respective ONU""s 4 to the OLT 2, in order to avoid collision among cells transmitted from the respective ONU""s 4, the OLT 2 transmits PLOAM (Physical Layer Operation, Administration, and Maintenance) cells to the ONU""s 4, respectively, and each of the ONU""s 4 transmits cells at relevant timings in ascending frames as specified by a transmission granting information included in the PLOAM cells. As such, ascending transmission speeds of the respective ONU""s 4 in the common PON system have been determined by a transmission period of the transmission granting information included in PLOAM cells to be transmitted from the OLT 2 to the respective ONU""s 4.
FIG. 9 shows frame formats of data to be transmitted on the PON system of FIG. 8.
As shown in FIG. 9, first and second PLOAM cells are included in a descending frame. The first PLOAM cell includes transmission granting informations for 1st through 27th cells in an ascending frame, while the second PLOAM cell includes transmission granting informations for 28th through 53rd cells in the ascending frame. Each of the PLOAM cells has a pay load including the contents as shown in the following Table 1.
The GRANTn (n=1 to 27) in Table 1 is a transmission granting information corresponding to an n-th cell of the ascending frame (it is noted that the GRANT27 of the second PLOAM cell is an idle GRANT which does not request any transmission from ONU). The ascending transmission speed of each of ONU""s 4 is allocated with a fixed rate based on a peak cell rate (PCR) of each communication channel, irrespectively of a relevant service category and an active/inactive state of the channel.
Each cell in the ascending frame has an overhead comprising a guard time field G for avoiding collision on the transmission line due to transmission fluctuation of ONU""s 4, a clock supplying field PR for supplying a clock in an ascending direction, and a field DL for detecting a heading of the ATM cell.
In the ATM-PON type communication system as described above, a band (ascending transmission speed) has been fixedly allocated to each communication channel on the PON, possibly resulting in such a situation that more bands on the PON are allocated than actually used, thereby causing waste of bands. To. avoid such a situation, there has been known a conventional technique for conducting effective allocation of bands, such as disclosed in Japanese Unexamined Patent Publication No. 10-224368 or No. 10-145386.
In an xe2x80x9cATM cell communication method in a passive double star networkxe2x80x9d disclosed in the former publication, a main device (corresponding to the OLT 2) exchanges control slots with subsidiary devices (corresponding to ONU""s 4), to thereby decide activated states of subsidiary devices. During activation of any one of the subsidiary devices, the main device allocates an ascending communication band to the activated one. Conversely, during inactivation of any one of the subsidiary devices, the main device allocates the band, which has been allocated to the inactivated one, to a variable capacity ATM service accommodated in another subsidiary device, to thereby realize an effective band allocation control.
In a xe2x80x9cconnection setting method in an ATM line concentratorxe2x80x9d disclosed in the latter publication, there is established a common path in, which data from subscribers flow between an ATM line concentrator and an ATM switchboard. When the ATM switchboard has received a call from a subscriber accommodated in the ATM line concentrator, the ATM switchboard notifies a band available in the common path to the ATM line concentrator, to thereby simplify setting treatment of connection.
However, there exists a problem as follows, in the aforementioned conventional techniques. Namely, in the method disclosed in the former Japanese Unexamined Patent Publication No. 10-224368, those bands, which have been allocated to the inactivated subsidiary devices, are allocated irrespectively of the traffic amounts (presence/absence of data, bands) of a virtual path (VP) of the variable capacity ATM service being a subject of allocation. Namely, the bands are allocated equally to those VP""s of variable capacity ATM service of less traffic amount and to those VP""s of variable capacity ATM service of large traffic amount, resulting in that an effective band allocation may not be fully realized. Further, in the method disclosed in the latter Japanese Unexamined Patent Publication No. 10-145386, there may be caused a situation that an actual traffic amount does not exist so much as that of the band established between the ATM line concentrator and the ATM switchboard such as due to congestion of the ATM network, resulting in waste of the difference between the established band and the actual traffic amount.
Meanwhile, there is a best effort type service which does not guarantee a user""s available communication speed in a congested state of a network in an ATM communication system. As categories of this service, there are known such as available bit rate (ABR) service, unspecified bit rate (UBR) service, and guaranteed frame rate (GFR) service.
The ABR service means a service in which an ATM network conducts a flow control in response to a congestion state of the network, such that a wide transmission band is allocated to a user terminal in a range between a minimum cell rate (MCR) and a peak cell rate (PCR) when a network resource is not fully used, and the thus allocated transmission band is notified to the user terminal, and a transmission restriction is imposed on a transmitting user terminal when a congestion has occurred in the ATM network. The UBR service means a service in which a band guarantee is not given, but data can be transmitted at a rate up to the PCR when the network resource is not fully used. The GFR service means a service in which the MCR is guaranteed, and data can be transmitted at a rate up to the PCR when the network resource is not fully used.
In the se best effort type services, the transmission speed is increased up to the PCR at the maximum in a situation where the ATM network is not fully used. How ever, in case of occurrence of congestion in the ATM network, the transmission speed is lowered up to the MCR at the minimum according to the flow control at the transmitting terminal of the ABR communication channel; lowered up to the MCR at the minimum at the transmitting terminal of the GFR communication channel; and lowered up to 0 (zero) at the minimum at the transmitting terminal of the UBR communication channel.
Meantime, on a PON, even with the best effort type service, ascending transmission speed of each of communication channels is allocated fixedly with a value equal to or larger than the PCR of the respective communication channels, such as irrespectively of: change of transmission speed due to communicating state such as of the ATM network and transmitting/receiving terminals; and change of an active/inactive state of the respective communication channels themselves. As such, this situation means that a transmission speed larger than actually required is allocated to each of the respective communication channels on the PON, and further that a needless a transmission speed is allocated to a communication channel through which no data cell is transmitted.
There will be now concretely explained a flow control of an ABR channel.
FIG. 10 exemplarily shows an example of transition of a transmission speed of an ABR channel.
As shown in FIG. 10, a transmitting terminal of ABR channel performs transmission at an allowed cell rate (ACR) as an allowed transmission speed represented by an ordinate. Namely, upon starting transmission, the transmission is performed at an initial cell rate (ICR) which is an initial value of the ACR. Then, when the transmitting terminal has received a backward resource management (B-RM) cell via an ATM network, if a CI (Congestion Indication) as a congestion indicator and an NI (No Increase) indicating no increase of the transmission speed, both contained in the B-RM cell, are simultaneously 0 (zero), there is calculated a new ACRxe2x80x2 (AC Rxe2x80x2=ACR+PCRxc3x97RIF) by multiplying PCR by a rate increase factor (RIF) and adding the obtained product to the current ACR. When this ACRxe2x80x2 is smaller than an explicit rate (ER) included in the received B-RM cell, it is possible to transmit data at the ACRxe2x80x2. When this ACRxe2x80x2 is larger than the ER, it is possible to transmit data at the ACR, by making ACR=ER.
Further, the CI of the received B-RM cell is 1, there is calculated a new ACRxe2x80x2 (ACRxe2x80x2=ACRxe2x88x92ACRxc3x97RDF) by multiplying ACR by a rate decrease factor (RDF) and subtracting the obtained product from the current ACR. When this ACRxe2x80x2 is smaller than the ER of the B-RM cell, it is possible to transmit data at the ACRxe2x80x2. When this ACRxe2x80x2 is larger than the ER, it is possible to transmit data at the ACR, by making ACR=ER. Further, when the NI of the received B-RM cell is 1, the transmission may be performed by maintaining the current ACR.
Since the transmission speed of the ABR channel is varied according to the aforementioned flow control such that the ascending transmission speed is fixedly allocated by the PCR on the PON, there is caused a problem that an unused region is left as shown by a shaded portion in FIG. 10.
The present invention has been carried out in view of the aforementioned points, and it is therefore an object of the present invention to provide an ascending transmission speed controlling method and a communication system in an ATM-PON (asynchronous transfer mode-passive optical network) system, in which ascending transmission speeds on the PON are dynamically allocated corresponding to actual transmission states of communication channels of a best effort type, to thereby effectively utilize respective bands.
To achieve the above object, the present invention provides an ascending transmission speed controlling method-in an ATM-PON system, in which the PON system has an optical line termination (OLT) connected to an ATM network and a plurality of optical network units (ONU) each of which is connected to a subscriber terminal to thereby send/receive cell information to/from the optical line termination, and accommodates a plurality of communication channels of best effort type in which a communication speed available by a subscriber is not guaranteed when the ATM network is congested; the method being for controlling transmission speeds of ascending cell informations to be transmitted from the plurality of optical network units to the optical line termination; the method comprising the steps of: identifying, by the optical line termination, whether the communication channels are in active states, respectively, based on receiving states of ascending cell informations from the optical network units, respectively; extracting, by the optical line termination, resource management (RM) cells included in descending cell informations from the ATM network; allocating, by the optical line termination, ascending transmission speeds on the PON to the communication channels, respectively, based on the identified active/inactive states of the communication channels, a congestion state of the ATM network indicated by the extracted resource management cells, and a maximum transmission speed on the PON; generating, by the optical line termination, ascending transmission grant signals to be transmitted to the optical network units according to the allocated ascending transmission speeds; and transmitting, by the optical network units, the ascending cell informations from the subscriber terminals to the optical line termination according to the ascending transmission grant signals from the optical line termination.
According to such a method, actual usage states of communication channels on the PON are identified and the congestion state of the ATM network is recognized by extracting resource management cells, at the optical line termination. Ascending transmission speeds on the PON are allocated to the respective communication channels based on the thus recognized information and a maximum transmission speed (band) on the PON, so that ascending transmission speeds of optical network units are controlled, respectively. In this way, bands on the PON are effectively utilized corresponding to an actual communication state of the network, so that the communication channels of best effort type can be accommodated effectively and flexibly.
The step of allocating ascending transmission speeds may allocates transmission speeds calculated corresponding to the congestion state of the ATM network represented by the extracted resource management cells, as ascending transmission speeds on the PON, respectively, to communication channels the service categories of which are available bit rate (ABR) service and which are in active states, and then, allocates the remaining ascending transmission speeds on the PON, as ascending transmission speeds on the PON, respectively, to communication channels the service categories of which are guaranteed frame rate (GFR) service or unspecified bit rate (UBR) service and which are in active states, according to the number of the corresponding communication channels.
According to such a method, there is conducted a flow control for communication channels of available bit rate service, according to information of the resource management cell. Further, ascending transmission speeds on the PON are allocated to communication channels of guaranteed frame rate service and unspecified bit rate service, corresponding to actual ascending transmission states. In this way, it becomes possible to increase ascending transmission speeds of communication channels in active states, and to avoid such a situation that transmission speeds are wastefully allocated to communication channels in inactive states.
The above method may further comprise the steps of: measuring ratios of actual transmission speeds relative to the transmission speeds allocated by the optical line termination as the ascending transmission speeds on the PON, for those communication channels of guaranteed frame rate service and unspecified bit rate service which are identified as active; and reducing the ascending transmission speeds allocated to the corresponding communication channels to allocate the reduced amount of speed to those communication channels the measured ratios of which are higher than predetermined ratios, when the measured ratios are lower than predetermined values, respectively.
According to such a method, it becomes possible to control the respective communication channels of guaranteed frame rate service and unspecified bit rate service, corresponding to actual ascending transmission speeds, thereby enabling effective utilization of bands on the PON.
The above method may further comprise the step of: inserting, by the optical line termination, resource management cells into ascending cell informations from the optical network units, respectively; and transmitting them to the ATM network, for those communication channels of guaranteed frame rate service and unspecified bit rate service which are identified as active; wherein the step of allocating ascending transmission speeds may allocates transmission speeds calculated corresponding to the congestion state of the ATM network represented by the extracted resource management cells, as ascending transmission speeds on the PON, respectively, to those communication channels of available bit rate service, guaranteed frame rate service and unspecified bit rate service which are in active states, respectively. Preferably, the step of allocating the ascending transmission speeds preferentially allocates ascending transmission speeds on the PON to communication channels of available bit rate service, respectively.
According to such a method, it becomes possible to control ascending transmission speeds on the PON also for communication channels of guaranteed frame rate service and unspecified bit rate service, respectively, according to a flow control by resource management cells, similarly to communication channels of available bit rate service.
Further, the step of allocating ascending transmission speeds may allocate to communication channels in inactive states, those ascending transmission speeds on the PON, which have been previously set corresponding to service categories of the communication channels and then, allocates ascending transmission speeds to communication channels in active states, respectively.
Concretely, the step of allocating ascending transmission speeds may allocate previously set initial cell rates (ICR) to those communication channels of available bit rate service which are in inactive states, as ascending transmission speeds on the PON, allocate previously set minimum cell rates (MCR) to those communication channels of guaranteed frame rate service which are in inactive states, as ascending transmission speeds on the PON, and allocate previously set substantially zero speed to those communication channels of unspecified bit rate service which are in inactive states, as ascending transmission speeds on the PON. Further, when a communication channel has shifted from an inactive state into an active state, the step of allocating the ascending transmission speeds may allocate an ascending transmission speed granted on the PON within a range between a previously set peak cell rate (PCR) or less and a minimum cell rate (MCR) or more if the communication channel is of the guaranteed frame rate service, and, allocate an ascending transmission speed granted on the PON within a range equal to or lower than a previously set peak cell rate (PCR) if the communication channel is of the unspecified bit rate service.
According to such a method, it becomes possible for subscriber terminals corresponding to communication channels in inactive states, to transmit ascending cell informations at any time.
In the above method, it is preferred that: the optical network units change the ascending transmission speeds at timings earlier by predetermined time lengths than timings at which ascending transmission speeds of the subscriber terminals are changed, respectively, when the optical network units increase the ascending transmission speeds according to the ascending transmission grant signals from the optical line termination, respectively, and the optical network units change the ascending transmission speeds at timings later by predetermined time lengths than timings at which ascending transmission speeds of the subscriber terminals are changed, respectively, when the optical network units decrease the ascending transmission speeds according to the ascending transmission grant signals from the optical line termination, respectively.
According to such a method, there can be avoided such a situation that cell informations transmitted from subscriber terminal to optical network unit are overflowed due to failure of reservation of band on the PON.
In the above method, when each of the optical network units accommodates a plurality of communication channels of different service categories, the each of the optical network units may distributingly write ascending cell informations transmitted from the subscriber terminals into a plurality of buffers corresponding to priority of the service categories, respectively, and transmits ascending cell informations stored in the buffers in an order of precedence from the highest priority, to the optical line termination according to ascending transmission grant signals from the optical line termination, respectively.
According to such a method, the transmission of ascending data cells from optical network units to the optical line termination can be effectively conducted, corresponding to precedence of the respective communication channels.
The present invention further provides an ATM-PON type communication system in which the PON system has an optical line termination connected to an ATM network, and a plurality of optical network units each of which is connected to a subscriber terminal to thereby send/receive cell information to/from the optical line termination, and accommodates a plurality of communication channels of best effort type in which a communication speed available by a subscriber is not guaranteed when the ATM network is congested, wherein the optical line termination comprises: an active state identifying device for identifying whether the communication channels are in active states, respectively, based on receiving states of ascending cell informations from the optical network units, respectively; a resource management cell extracting device for extracting resource management cells included in descending cell informations from the ATM network; a transmission speed controlling device for allocating transmission speeds on the PON to the communication channels, respectively, based on active/inactive states of the communication channels identified by the active state identifying device, on a congestion state of the ATM network indicated by the resource management cells extracted by the resource management cell extracting device, and on a maximum transmission speed on the PON; and a transmission grant signal generating device for generating the ascending transmission grant signals to be transmitted to the optical network units according to the ascending transmission speeds allocated by the transmission speed controlling device, respectively, wherein each of the optical network units comprises an ascending transmission device for transmitting the ascending cell informations from the subscriber terminals toward the optical line termination according to the ascending transmission grant signals from the transmission grant signal generating device.
Concerning the above ATM-PON type communication system, the optical line termination may include a speed measuring device for measuring actual ascending transmission speeds of those communication channels which are identified as active by the active state identifying device, and the transmission speed controlling device may adjust ascending transmission speeds on the PON allocated to the communication channels corresponding to actual transmission speeds measured by the speed measuring device, respectively.
Alternatively, the optical line termination may include a resource management cell generating device for inserting resource management cells into ascending cell informations from the optical network units, respectively, and transmitting them to the ATM network, for those communication channels of guaranteed frame rate service and unspecified bit rate service which are identified as active by the active state identifying device.
Further, it is preferred that when the ascending transmission device of each of the optical network units accommodates a plurality of communication channels of different service categories, the ascending transmission device of the each of the optical network units comprises: a plurality of buffers corresponding to priority of the service categories, respectively; a channel distributing part for distributingly writing ascending cell informations transmitted from the subscriber terminals into the buffers corresponding to the service categories; and a buffer selecting part for transmitting ascending cell informations stored in the buffers in an order of precedence from the highest priority, to the optical line termination according to ascending transmission grant signals from the optical line termination, respectively.